1. Field of the Invention
The invention relates to an apparatus for the manufacture of a plastic pipe having transverse profile features.
2. Background Art
In an apparatus of the generic kind known from U.S. Pat. No. 4,900,503 the half shells arranged in continuous sequence one after the other are interconnected by means of fish plates to form a chain. At the beginning of the molding path, a driving pinion engages with a denticulation provided on the upper side of the half shells, the driving pinion simultaneously driving the half shells associated with each other in pairs. An injection head of an extruder is arranged before the molding path and injects a thermally plastic tube of plastic material into the molding chamber where, between a calibrating mandrel and mold recesses provided in the half shells, it is formed into a pipe having transverse profile features, for instance ribs. The mold recesses are acted upon by vacuum through vacuum lines formed in the half shells. They are connected to a negative pressure source by way of suction connections provided in the machine bed. The guide devices which press the half shells together on the molding path have guide rails arranged on both sides of the molding path and to which guide rollers are rotatably mounted, which in turn rest against the outside of the half shells. One guide rail is fixed to the machine bed, while the other guide rail is pressed by springs against the half shells passing though the molding path. This is necessary in order to ensure that the half shells adjoining in pairs mutually abut closely to each other. This basic structure of the apparatus has been known for a long time from U.S. Pat. No. 3,776,679.
Considerable changes of pressure between the calibrating mandrel and the half shells arise due to fluctuations in the performance of the extruder, changes in the viscosity of the plastic material as a result of variations of temperature in the extruder or as a result of fluctuating cooling conditions on the molding path and as a result of frequently changing quantities of plastic material needed for the manufacture of the pipe along a section in the length of the plastic pipe, for instance when a rib is formed or when smooth but rather thick-walled sockets or spigots are injection-molded. These fluctuations of pressure cannot be taken up by the springs mentioned which are commonly in the form of packages of pre-stressed disk springs.
To counter this problem it has become known from EP 0 359 088 A2 to provide a compensation chamber where the injection head passes into the calibrating mandrel and to fill it with a fluid under pressure which, in turn, can yield. Any excess plastic material may escape into this chamber. This implies considerable sealing problems.
In this connection, DE 38 10 915 C1 teaches to rigidly interlock the half shells abutting in pairs on the molding path, the interlocking being produced at the beginning of the molding path and released at the end of the molding path by corresponding stops being triggered.
EP 0 240 838 A2 teaches a fundamentally similar solution in which the half shells abutting in pairs on the molding path are mutually interlocked by conical pins engaging with the half shells. These pins are removed at the end of the molding path.